1. Field of the Invention
The present invention relates to a phase detection system, and more particularly, to a system for detecting the phase of a color burst signal component of a video signal, which is suited for use in a video disk player system.
2. Description of Background Information
In connection with video disk player systems, playback operations of a CAV (Constant Angular Velocity) disk will be discussed hereinafter. In CAV disks, a composite video signal is recorded in such a manner that a video signal corresponding to one frame is recorded in every one of the recording tracks in the case of the disk having concentric recording tracks or in every circular section of the recording tracks in the case of a disk having a spiral recording track.
The relation between the frequency fsc of the color subcarrier signal and the frame frequency fF in the NTSC system is determined by the following equation: EQU fsc=(455/2).times.525.times.fF.
This equation reflects the fact that the subcarrier signals recorded in a first frame and an adjacent second frame are 180.degree. out of phase.
Since the video signal corresponding to one frame is recorded in one of the recording tracks or one circular section of the recording track, the phase difference between the subcarrier signals recorded in two adjacent recording track sections is equal to 180.degree..
In the case of the recording disk having a recording track of the spiral form, the continuity of the phase alternation condition of the subcarrier signal is maintained during a normal playback mode in which the recorded video format signal is picked up from an innermost part of the recording track to an outermost portion thereof. However, in addition to this normal playback mode, video disk playback systems are generally constructed to perform the so called SCAN playback mode for performing a fast speed playback operation in which an information reading point of a pickup device is moved in a radial direction across a plurality of the recording track portions. During such a SCAN playback mode, a tracking servo system of the playback system which has a tracking actuator is actuated to place the information reading point of the pickup device at each of the recording track portions on the recording disk in sequence, during the radial movement of the pickup device. However, since the movement of the tracking actuator is limited within a predetermined movable range, it is necessary to stop the tracing operation of the tracking servo system so as to put the position of the tracking actuator back to a center of the movable range.
During this period in which the tracing movement of the pickup device is stopped, the light beam for reading information moves across a plurality of recording track portions. Now, the phase relation between the color subcarrier signals of the video signals picked up before and after the crossing of the recording track portions by the pickup means will be discussed. If the number of circular track portions across which the information reading point moves is even, there will be no phase difference between the color subcarrier signals, and the continuity of the video format signal is maintained. On the other hand, if the number of track portions across which the information reading point moves is odd, the phase difference between the color subcarrier signals before and after the movement of the information reading point becomes equal to 180.degree.. Therefore, the continuity of the video format signal is not maintained under this condition. If a playback video signal under this condition is monitored by a video monitor system, the proper hue of the video information is not reproduced when the phase of the color subcarrier signal is inverted. Thus, the reproduction of the color component of the video information becomes unstable.
One way to solve this problem is to count the number of recording track portions across which the information reading point moves, and to invert in phase the color video signal depending on whether the number of recording track portions is even or odd. However, it is generally not possible to count the number of recording track portions precisely due to noise.
Further, in the case of a picture information reproducing system using a frame memory, video signals having the same waveform are supplied from the frame memory during a freeze frame playback mode. In that case, a compensation operation is necessary to maintain the continuity of the color subcarrier signal.